Composite base material and adhesive film

ABSTRACT

To provide an adhesive film that can be easily released leaving substantially no adhesive residue and without damaging the adherend surface when released from an adherend. A base material comprising at least a first resin film with a breaking elongation in the lengthwise direction of 300 to 1500% and a breaking strength of at least 2 kg/12 mm, and a second resin film with a lower breaking elongation and a higher flexural modulus than the first resin film is used. The second resin film can break before the first resin film when the base material is stretched.

DETAILED DESCRIPTION OF THE INVENTION

1. Technical Field of the Invention

The present invention relates to a composite base material and anadhesive film using the same. The adhesive film of the present inventioncan be released after attachment to adherends, allowing its release withstretching of the film (known as “stretch release”), thus rendering itadvantageous not only for office use, but also particularly forindustrial use.

2. Prior Art

Various types of tacky tapes and adhesive tapes have been proposed andimplemented that can be suitably used for a diverse range of purposes.In Japanese National Publication (Kohyo) No. 6-504077, for example,there is disclosed a removable adhesive tape comprising a backing andpressure-sensitive adhesive borne on at least one main surface thereof,characterized in that the backing has a breaking elongation of about 150to 1200% in the lengthwise direction, an elastic recovery of less thanabout 50% after elongation and a Young's modulus of at least from about175.8 kg/cm² to less than about 5097 kg/cm², and the tape can bestrongly bonded to the base and removed after being stetched at an angleof no greater than about 35° from the surface of the base. This adhesivetape has the effect of being peelable without damaging the adherend orholder, by release within a specified angle with respect to the adhesionside. However, the adhesive tape has a few drawbacks, in that when thebacking is formed from a single-material film such as LLDPE (linearlow-density polyethylene), HDPE (high-density polyethylene), EVA(ethylene-vinyl acetate copolymer), PP (polypropylene), TPE(thermoplastic elastomers) or the like and the tape is finished for useas an office supply tape, the entire tape has a lighter gauge, i.e. thethickness of the film itself is about 10 to 250 μm and in practice about20 to 100 μm, so that the low Young's modulus of the material in theunstretched state after fabrication results in a lack of stiffness, thusgreatly complicating operations by end-users, such as pulling out andpasting of the tape. The mechanical strength of the adhesive tape,including its stiffness, can be improved by stretching of the raw filmmaterial, but when the stiffness is enhanced to a practically usablelevel by stretching, the tensile strength also increases in excess,while the elongation is also lowered.

In Japanese National Publication (Kohyo) No. 9-502213 there is disclosedan adhesive tape comprising a support and a first pressure-sensitiveadhesive composition coated onto at least one surface of the support,wherein the support includes a foamed polymer layer with a thickness of30 to about 1000 mils (milli-inches) and the support has a breakingelongation of about 50% to about 1200% in the lengthwise direction,while the tape can be firmly bonded to substrates but can also bereleased therefrom when stretched at an angle of no greater than about35° from the surface of the substrate, without breaking the supportbefore the tape is released from the substrate and without leaving anysubstantial residue of the pressure-sensitive adhesive on the substrate.Since this adhesive tape includes a foamed polymer layer as the supportit is also adaptable to certain rough surfaces, and it does not limitthe freedom of the user. However, the adhesive tape is quite thick, asis obvious from the thickness of the polymer foam layer described above,and therefore while it is convenient for attachment of paintings and thelike onto walls and other surfaces, its thickness must be reduced inorder to be used as office supply adhesive tape, or as industrialadhesive tape for attachment of masking sheets and the like.

Problems to be Solved by the Invention

It is an object of the present invention to overcome the disadvantagesof conventional tacky tapes and adhesive tapes mentioned above, that is,to provide an adhesive film that can be reliably attached to adherendsand released when necessary, which can be easily released from adherendsleaving substantially no adhesive residue and without damaging theadherend surfaces, and which is easy to handle by users.

It is another object of the invention to provide a film-like basematerial useful in production of the adhesive film of the presentinvention.

Means for Solving the Problems

In one aspect thereof, the invention provides a film-like composite basematerial comprising at least a first resin film with a breakingelongation in the lengthwise direction of 300 to 1500% and a breakingstrength of at least 2 kg/12 mm, and a second resin film with a lowerbreaking elongation and a higher flexural modulus than the first resinfilm, characterized in that the second resin film breaks before thefirst resin film when stretched.

In another aspect thereof, the invention provides an adhesive filmcharacterized by comprising a film-like composite base materialcomprising at least a first resin film with a breaking elongation in thelengthwise direction of 300 to 1500% and a breaking strength of at least2 kg/12 mm, and a second resin film with a lower breaking elongation anda higher flexural modulus than the first resin film, and an adhesivelayer formed on at least one side of the composite base.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a preferred embodiment of acomposite base material according to the invention.

FIG. 2 is a cross-sectional view of another preferred embodiment of acomposite base material.

FIG. 3 is a cross-sectional view of a preferred embodiment of anadhesive film according to the invention.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

The film-like base material of the invention is a composite basematerial which is an integral combination of at least two resin filmswith different properties, preferably by lamination. The first resinfilm is highly extensible and has high breaking strength, andspecifically, it has a breaking elongation in the lengthwise directionof 300 to 1500% and a breaking strength of at least 2 kg/12 mm. Thesecond resin film has a lower breaking elongation and a higher flexuralmodulus than the first resin film. The first resin film can thereforeimpart high extensibility and high breaking strength to the basematerial. The second resin film used in combination with the first resinfilm improves the stiffness, while it breaks before the first resin filmdue to its excellent toughness, thus allowing it to impart a “cut andstretch” release mode to the base. In other words, when the compositebase material having this construction is stretched in the lengthwisedirection, the second resin film breaks earlier at the initial stage ofstretching, and after the second resin film has broken it is possible tocontinue to stretch first resin film with its high extensibility andhigh breaking strength, so that even when the composite base material isused as a substrate of an adhesive film, the adhesive layer can continueto adhere to the base material allowing adequate release with virtuallyno residual adhesive on the adherend.

FIG. 1 is a cross-sectional view showing a preferred embodiment of acomposite base material according to the invention. The composite base10 in FIG. 1 illustrates a two-layered structure, with a first resinfilm 1 and a second layer film 2 laminated in an integral manner.

In the composite base material of the invention, the first resin filmmay be formed of various different resin materials, but it must as anessential condition exhibit a breaking elongation in the lengthwisedirection of 300 to 1500% and a breaking strength of at least 2 kg/12mm. This high extensibility and high breaking strength are particularlynecessary for easy release of the adhesive film without damage to theadherend surface when the film is released, and to avoid any substantialresidue of the adhesive after release.

Resinous materials that may be advantageously used to form the firstresin film include, but are not restricted to, the following examples:polyolefins such as high-density polyethylene (HDPE), low-densitypolyethylene (LDPE), linear low-density polyethylene (LLDPE), linearultralow-density polyethylene (LULDPE) and polypropylene (PP); polyvinylpolymers such as polyvinyl chloride (PVC) and polyvinyl acetate (PVA);polyolefin-based copolymers such as ethylene-methacrylic acid copolymer(EEMA) and ethylene-vinyl acetate copolymer (EVA); block copolymers suchas acrylate polymer and styrene-isoprene-vinyl acetate copolymer;thermoplastic elastomers (TPE), and the like. These resinous materialsmay be used alone or in combinations of two or more resinous materials.

The resin film composed of the resinous material described above may bea film in any of various forms. As examples of suitable films there maybe mentioned nonwoven fabric films, woven fabric films, porous films andfoam films. These films may, if necessary, be used in combination withother forms of film(s) for preparation into a single resin film.

The second resin film used in combination with the first resin film inthe composite base material of the invention must as an essentialcondition be composed of a resin with a lower breaking elongation and ahigher flexural modulus than the first resin film. This resin film cantherefore compensate for the inadequate stiffness of the first resinfilm, in order to achieve the above-mentioned special release mode, or“cut and stretch”.

Resinous materials that may be advantageously used to form the secondresin film include, but are not restricted to, the following examples:polymethyl methacrylate, polycarbonate, polyethylene terephthalate,polystyrene, polyimide, cellulose acetate and the like. These resinousmaterials may be used alone or in combinations of two or more resinousmaterials.

The resin film composed of the resinous material described above may bea film in any of various forms, as described for the first resin filmabove. As examples of suitable films there may be mentioned nonwovenfabric films, woven fabric films, porous films and foam films. Thesefilms may, if necessary, be used in combination with other forms offilm(s) for preparation into a resin film.

The second resin film may, if necessary, contain an organic or inorganicfiller dispersed in the film. For example, inorganic fillers includecalcium carbonate, titanium oxide, silica and the like. Such fillers areuseful for enhancement of the film stiffness, lowering the elongationyield point to prevent the film from being easily cut, and lowering thebreaking strength.

While the organic or inorganic filler included in the second resin filmmay be of any form or size, it is preferably in the form of globularparticles, needle-like crystals or a similar form, and while its sizemay vary widely, it is usually in the range of about 1 to 20 μm. Forexample, in the case of needle-like whiskers, they are preferablyrelatively fine with a length of preferably in the range of about 1 to15 μm, and more preferably in the neighborhood of about 10 μm. Thediameter of such needle-like whiskers will usually be in theneighborhood of about 0.1 μm. If the size of the filler used here issmaller than 1 μm it will be necessary to use a large amount of fillerto achieve the effect of the added filler, while if it is over 20 μm anadverse influence may be exhibited against formation of the film and theother desired effects.

The filler such as described above may be included in the resin film inan amount within a wide range of selection, but in most cases it ispreferably added in the range of about 5 to 50 wt %. If the amount offiller added is less than 5 wt % the effect of addition may not beexhibited, and if it is added at greater than 50 wt % an adverseinfluence may be exhibited against formation of the film and the otherdesired effects. In most cases, the filler is preferably added in theneighborhood of about 10 wt %.

The composite base material of the invention has a “cut and stretch”feature as the first resin film stretches after the second resin filmhas broken; however, if the breaking strength of the second resin filmis higher than the yield stress of the first resin film, not only willthere be a feeling of resistance upon stretching, but breakage of thesecond resin film will simultaneously increase the possibility ofbreakage of the first resin film.

In order to more successfully exhibit the “cut and stretch” performance,it is effective to reduce the layer proportion of the second resin film,so long as this does not cause a problem for the working step or forattachment when used as an adhesive tape. However, reducing the layerproportion of the second resin film will lower the breaking strength andreduce the stiffness. The layer proportion of the second resin film canbe reduced by adding needle-like whiskers or the like to the secondresin film. This can improve the stiffness while keeping a low breakingstrength. The overall thickness of the composite base material cantherefore be decreased by reducing the layer proportion.

The composite base material of the invention may, if necessary, becolored with a desired color. Coloration can improve the manageabilityof the finally obtained adhesive tape. The coloration may be carried outfor either the first or second resin film, but it is usually preferredfor the second resin film. The color for the coloration may be a singlecolor, or two or more different colors may also be combined for designenhancement.

The composite base material of the invention may, if necessary, have amultilayered structure of three or more layers, and usually athree-layered structure such as shown in FIG. 2 is preferred. Thecomposite base material 10 of FIG. 2 is a modified form of the compositebase material shown in FIG. 1, having a construction wherein the secondresin film 2 is sandwiched by first resin films 1. By thus sandwichingthe second resin film 2 with two sheets of first resin films 1, it ispossible to obtain an effect of reinforced stiffness, firmness and easeof use beyond that obtained with a single first resin film. Furthermore,since the second resin film 2 is enclosed, scattering of fine brokenpieces or the added filler powder into the surrounding area can beprevented even when it breaks first.

The composite base material of the invention may also include otheradditional layers, not shown, employed in the field of adhesive filmswhen necessary, such as cushion layers, release layers and the like,that do not adversely influence the function and effect of the inventionand that can offer additional effects.

The thickness of the composite base material of the invention may bevaried within a wide range depending on the intended use, but in mostcases the total thickness of the first and second resin films ispreferably in the range of about 10 to 250 μm, and more preferably inthe range of about 30 to 100 μm. If the thickness of the composite basematerial is under 10 μm, it may not be possible to maintain sufficienttensile strength or the function as an adhesive film base may be lost;conversely, if it is over 250 μm the higher tensile strength maysubstantially complicate release of the tape from adherends, and theeffect of producing a light-gauge adhesive film may be impaired.

The composite base material of the invention can be produced usingtechniques commonly employed for fabrication of conventional compositefilms. For example, it may be produced by simultaneously extruding theresin materials for the first and second resin films through anappropriate die, and then drawing them. Alternatively, each resin filmmay be prepared beforehand and then laminated and fusion bonded into anintegral body. The respective resin films may be bonded with an adhesivein some cases.

In addition to the composite base material, the present invention alsoprovides an adhesive film employing the composite base material of theinvention. That is, an adhesive film according to the invention is anadhesive film characterized by comprising an adhesive layer formed on atleast one side of the composite base material of the invention.

A typical embodiment of the adhesive film of the invention is shown inFIG. 3. In the case of the adhesive film 20 shown in the drawing, theadhesive layer 4 is formed on the side of one first resin film 1 of thecomposite base material 10 of the invention which is illustrated in FIG.2 and explained above, via a primer layer 3.

In the adhesive film of the invention, the adhesive layer may be formedfrom any conventional adhesives in the field of adhesive films. Asexamples of appropriate adhesives there may be mentioned acrylic-basedadhesives, such as copolymer of isooctyl acrylate and acrylic acid,synthetic rubber-based adhesives such as polysilicone, polyisoprene,polybutadiene, styrene-isoprene-styrene copolymer, natural rubber-basedadhesives, and the like. Acrylic-based adhesives and syntheticrubber-based adhesives are particularly useful.

These adhesives may be coated and dried onto one or both sides of thecomposite base material using a well-known technique such as spincoating, screen printing or the like. It is usually preferred for theadhesive-coated side of the composite base material to be pretreated byprimer treatment, etc. according to a well-known technique, in order topromote coating and attachment of the adhesive. The thickness of thedried adhesive layer may vary widely depending on the properties of thedesired adhesive film and other factors, but it will usually be in therange of about 10 to 100 μm. The surface of the adhesive layer may alsohave a release liner or the like, or be subjected to other releasetreatment, as is commonly done in the technical field, in order toprotect it from undesired attachment.

The adhesive film of the invention must have a 180° peel strength of atleast 160 g/25 mm as measured with a peel rate of 300 mm/minute. This isbecause if the 180° peel rate is any lower it may not be possible tosimultaneously satisfy the important effects of the invention, which aregreater adhesive strength, lack of damage to adherends upon release, andsubstantially no residue of the adhesive after release.

When the adhesive film of the invention is attached to an adherend, itcan be simply and easily peeled off by stretching the side of theadhesive film with no adhesive layer at an angle within 35° of theattachment surface as it is peeled off after attachment to the adherend;there is virtually no inconvenient residue of the adhesive on the filmattachment surface of the adherend.

The adhesive film of the invention may be provided in a variety ofdifferent forms. For example, the adhesive film of the invention may belaminated without using a release sheet, such as a “Post-It™”-typelaminate wherein the adhesive is coated in the same direction. Theadhesive film of the invention may also be laminated by coating theadhesive in different directions. Alternatively, the adhesive film ofthe invention may be in the form of a rectangular sheet with theadhesive layer formed on only one side of the composite base material,and made into a concertina laminate where the portions with no adhesivelayer are alternately stacked. Such a laminate can be housed in anappropriate dispenser to provide a “pop-up” construction, so that eachsheet can be pulled out for use as necessary.

The adhesive film of the invention can be advantageously used in avariety of fields. As examples of suitable uses for the adhesive filmthere may be mentioned its use as common office supply adhesive tape,and its use as masking tape for closing of the ends of masking sheets atworking sites, construction sites and the like.

EXAMPLES

The present invention will now be further explained referring to theexamples. It is to be understood, however, that the invention is in noway limited by these examples.

Example 1

The following commercially available stock materials were provided asmaterials for the first and second resin films. The properties of eachfilm are also described.

First Resin Film:

Linear Low-density Polyethylene (LLDPE)

Available from Mitsui Chemical Co. under the product name “Ultzex 2520F”

Lengthwise breaking elongation: 550% (total for top and bottom layers)

Breaking strength: 3.3 kg/12 mm (total for top and bottom layers)

Second Resin Film:

Polymethylmethacrylate (PMMA)

Available from Mitsubishi Rayon Co. under the product name “Acrypet MF”

Lengthwise breaking elongation: 10%

Breaking strength: 1.2 kg/12 mm

The polymethylmethacrylate (PMMA) and the linear low-densitypolyethylene (LLDPE) were co-extruded with a T-die to fabricate acomposite film with a three-layered structure (LLDPE/PMMA/LLDPE=26 μm/8μm/26 μm). The stiffness of the resulting composite film was measuredand found to be 0.82 g/12 mm. In this example, the stiffness wasmeasured by the following procedure.

Measurement of Stiffness

Measuring device: Heart loop stiffness meter

Measuring procedure:

The composite film is cut into a thin strip test piece with a width of ½inch (about 25 mm) and a length of about 15 to 18 cm, and this is set onthe table of the measuring device with both ends of the test pieceanchored with a magnet jig. The measuring device switch is turned on andboth magnet jigs are automatically slid to the center of the table toform a loop with the test piece. After the test piece loop has beenformed, the table is laid on its side while a sensor mounted on themeasuring device is moved toward the loop at a rate of 200 mm/minute tocrush the test piece loop. The maximum value of the stress at the pointthe loop is crushed by the progressing sensor is recorded as thestiffness.

The obtained composite film was used as a base material, and wassubjected to primer treatment on one side. The primer used was asolution obtained by diluting chlorinated polypropylene (commerciallyavailable as “Hardlen B-13” (product name of Toyo Kasei Co.)) withtoluene to a 2% solid concentration. After hand coating the primer ontothe surface of the base material, an adhesive resin comprising isooctylacrylate-acrylic acid (95:5) copolymer was transferred to a filmthickness of 5 μm by a screen printing method, and dried. Uponmeasurement of the 180° peel strength of the resulting adhesive film, itwas found to be 450 g/25 mm when measured at a peel rate of 300 mm/min.For this example, the 180° peel strength of the adhesive film wasdetermined according to the following procedure.

Measurement of 180° Peel Strength

Measuring apparatus: Slip/peel tester (“Model 3M90”, product name ofInstrumentors Inc., U.S.)

Measuring procedure:

A 50-mm wide×150-mm long stainless steel test panel was prepared and theregion within 25 mm of the end of the panel was coated with maskingtape. Also, a composite film cut to a width of ½ inch (about 25 mm) anda length of about 15 cm was prepared as a thin test strip. The preparedtest strip was attached to the surface of the test panel with apressure-sensitive adhesive so that the end portion of the test stripoverlapped the masking tape. The bond between the test panel and thetest strip was strengthened by reciprocally moving a 4.5-kg weightroller over the test strip after attachment. One end of the test strip(the portion not adhered to the test panel) was then separated from themasking tape by hand, and the test strip was stretched at a 180° peelangle and a rate of 305 mm/min. The peel length of the test strip was127 mm, but for a more accurate measurement the peel data for the first25 mm were not used. The average peel strength measured for the rest ofthe peel length was recorded as the 180° peel strength.

After also attaching the obtained adhesive film onto a plain paper, aportion of the non-attached side of the adhesive film was stretched atan angle within 35°, and it was possible to easily release the adhesivetape without breaking the plain paper and without producing anysubstantial residue after the release. The adhesive tape felt hard atthe first stage of stretching, believed to be due to the PMMA film inthe middle, but this film broke shortly thereafter. Stretching was thenfacilitated due to the highly extensible LLDPE films on the outside.

Comparative Example 1

The procedure described in Example 1 above was repeated, but, forcomparison, in this example, a 60-μm thick single-layer film consistingof LLDPE alone was used instead of the 60-μm thick three-layeredstructure composite film of LLDPE/PMMA/LLDPE. The stiffness of thesingle-layer film was measured by the same method described in Example 1and confirmed to be 0.52 g/12 mm, which was less than about ⅔ of thestiffness of the composite film of Example 1.

Example 2

For evaluation of the effect of adding a filler to the second resinfilm, in this example, needle-like whiskers were added to the PMMA filmused as the second resin film in Example 1, and the breaking strength ofthe film was measured. The PMMA film used here was a single layer (15 μmthickness), and the needle-like whiskers were barium titanate whiskers(“TISMO”, product name of Otsuka Chemical Co.) having a diameter ofapproximately 0.1 μm and a length of about 10 μm, which were added to acontent of 10 wt % (with respect to the total weight of the film).

The breaking strengths of the PMMA film (containing no whiskers) and thePMMA film (containing whiskers) were measured, giving the followingresults.

PMMA film 250 g/12 mm Whisker-containing PMMA film 150 g/12 mm

The results of the measurement demonstrate that it was possible toconsiderably reduce the breaking strength by using a resin film obtainedby adding whiskers to a PMMA film.

Effect of the Invention

As explained above, by using the base material described above accordingto the present invention it is possible to provide an adhesive film thatcan be reliably attached to adherends, that can be easily released fromadherends in a special release mode of “cut and stretch”, leavingsubstantially no adhesive residue and without damaging the adherendsurface or holder when released from an adherend, and that is easy tohandle by users. The present invention also provides a composite basematerial that can guarantee sufficient firmness even with a smallthickness.

Explanation of Symbols

-   1 first resin film-   2 second resin film-   3 primer layer-   4 adhesive layer-   10 composite base material-   20 adhesive film

1. A film-like composite base material comprising at least a first resinfilm with a breaking elongation in the lengthwise direction of 300 to1500% and a breaking strength of at least 2 kg/12 mm, and a second resinfilm with a lower breaking elongation and a higher flexural modulus thansaid first resin film, said second resin film including a needle-likewhiskers filler, characterized in that said second resin film breaksbefore said first resin film when stretched.
 2. A composite basematerial according to claim 1, characterized in that said first resinfilm is laminated on both sides of said second resin film.
 3. Acomposite base material according to claim 1 or 2, characterized byhaving a thickness of 10 to 250 μm.
 4. An adhesive film characterized bycomprising a film-like composite base material comprising at least afirst resin film with a breaking elongation in the lengthwise directionof 300 to 1500% and a breaking strength of at least 2 kg/12 mm, and asecond resin film with a lower breaking elongation and a higher flexuralmodulus than said first resin film, and an adhesive layer formed on atleast one side of said composite base material, wherein said secondresin film includes a dispersed filler in a form of needle-likewhiskers, characterized in that said second resin film breaks beforesaid first resin film when stretched.
 5. An adhesive film according toclaim 4, characterized in that said adhesive layer is formed from anacrylic adhesive or synthetic rubber adhesive, and the 180° peel forceof said adhesive film is at least 160 g/25 mm when measured at a peelrate of 300 mm/minute.
 6. An adhesive film according to claim 4, whereinsaid adhesive layer is configured to be released from an adherend whensaid adhesive layer is stretched to elongate said first resin film andto break said second resin film.
 7. An adhesive film according to claim4, wherein said needle-like whiskers have a length in the range of about1 to 15 μm.
 8. An adhesive film according to claim 4, wherein saidneedle-like whiskers have a diameter of about 0.1 μm.
 9. An adhesivefilm according to claim 4, wherein said adhesive film is configured suchthat when applied to an adherend and subjected to a pulling force, saidsecond resin film initially breaks followed by stretching of said firstresin film.
 10. A method of using an adhesive film, said methodcomprising: providing an adhesive film as defined in claim 4; adheringsaid adhesive layer to an adherend; and applying a pulling force to oneend of said adhesive film to remove said adhesive film from saidadherend, characterized by: i) said second resin film initially breakingin response to said pulling force, ii) said first resin film stretchingin response to said pulling force after said second resin film hasbroken to effectuate stretch release of said adhesive film from saidadherend.